| 摘要 |
In a spacecraft (10) having devices (28) which are operative in plural modes, particularly an earth sensor operative in a north hemispherical inhibit mode, a south hemispherical inhibit mode, and a normal mode, a watchdog circuit (38) is employed to detect the presence of a mode command issued to the device. The command may be issued by a station (24) on the earth (14), and communicated via a telemetry link (26) to the spacecraft. The watchdog circuit may be part of a central processing unit (CPU) (22) with a computer on board the spacecraft, the computer serving to perform various data processing operations for the mission of the spacecraft. The watchdog circuit (38) stores the mode command and then repeats the command during a succession of repetitions. Thereby, in the event that the device, particularly the earth sensor, is momentarily disabled by an electrostatic discharge (ESD) event, wherein digital control circuitry of the earth sensor may shift the sensor to a different mode of operation, the next repetition of the mode command from the watchdog circuit reinstates the desired mode of operation. The watchdog circuit and the CPU are radiation hardened and may be enclosed by a Faraday shield (36) to render them substantially impervious to the radiation. <IMAGE> |
